1. Field of the Invention
The present invention relates to electroacoustic transducers whose vibrating element is formed of a membrane which comprises a piezoelectric layer with electrodes and is held in position at its periphery in an insert case.
2. Description of the Prior Art
This type of insert has already been known for a long time and is described for example in the patent No. FR-A-2 511 570, patent No. FR-A-2 337 480 or in the patent application No. JP-58-202699.
In known inserts, the case comprises a base body with a central cavity whose periphery forms a bearing surface for the periphery of the piezoelectric material layer membrane; the case further comprises a cover connected by its periphery to the base body with an inner recess communicating with the outside through at least one orifice; the periphery of the cover comprises an annular internal relief bearing on the membrane periphery so as to hold it applied against the periphery of the cavity of the base body.
The central cavity of the base body defines, with the piezoelectric membrane, an internal base cavity. The internal recess of the cover defines, with the piezoelectric membrane, an inner cover cavity. The piezoelectric material layer separating two conducting surfaces forming the electrodes forms an electric capacitor. The piezoelectric transducer effect causes on the electrodes an electric charge induced by the mechanical stresses undergone by the piezoelectric material layer.
In some cases, a piezoelectric membrane is used formed of a thin metal film forming the base substrate of the membrane and on which at least one piezoelectric ceramic layer is fixed. The external face of the piezoelectric ceramic is metallized, so as to form a first electrode, the substrate forming a second electrode. In other cases, the membrane is formed of a thin electrically insulating film, to which is fixed at least one piezoelectric ceramic layer, the two faces of which are metallized and form the electrodes.
The electrodes must be electrically connected to output terminals of the insert case. When the membrane comprises an electrically conducting substrate, one of the connections may be provided by a conductor soldered to the substrate. The second electrode, formed by the external metallized face of the piezoelectric material layer, is connected by a metal conductor one end of which is soldered to said metallized face.
During manufacture of such known inserts, the formation of the electric connections is a particularly delicate and expensive operation. In fact, the membranes are fragile elements, because their thickness is very small, about 100 to 200 microns. To solder such elements is difficult and requires expensive means. It must be further considered that the surface on which the conductors are to be soldered are not always easily accessible, more particularly in the case of membranes having several piezoelectric material layers. Such membranes with two piezoelectric material layers are for example described in the German patent application No. DE-A-3 309 851: a base substrate in the form of a thin metal or insulating film has, on both its faces, a piezoelectric ceramic layer whose two faces are metallized and form electrodes. The two piezoelectric elements must be connected electrically in parallel. It will be readily understood that with such a membrane structure the connections are particularly delicate to form.
An additional difficulty resides in the fact that the membrane must be held in position along its periphery by means providing sealing. The sealing is required for the correct acoustic operation of the inner base cavity and of the inner cover cavity of the insert. The Applicant has discovered that such sealing is not compatible with the passage of soldered electric conductors providing the electric connection between the electrodes and the output terminals of the insert.
Another problem met with in known inserts concerns the presence of solder for connecting the electric conductors electrically to the electrodes. The heating, required for soldering, locally degrades the piezoelectric effect in the piezoelectric material. Furthermore, the solder, as well as the conductor, increase the overall mass of the mobile membrane and lead to an appreciable loss of efficiency, and to the appearance of acoustic resonance phenomena.
The document GB-A-2 046 554 describes an electroacoustic insert with piezoelectric membrane in which the electric connections are provided without solder. In this document, the membrane substrate comprises a peripheral zone of radial excrescence, off-centered radially and projecting form the zone defined by the bearing surfaces and the seal. A conducting track, insulated by an insulating layer, connects the radial excrescence zone of the substrate to the electrode, so as to provide the electric connection. The excrescence zone extends outside the case and is intended to receive directly the external electric connection means. At the passage through the case, the radial excrescence of the substrate is embedded in the case and so is held rigidly. Such a solution avoids the use of solder but has several drawbacks, and particularly: embedment of the peripheral excrescence at its passage through the wall of the case disturbs the mechanical properties and the movements of the insert membrane, limiting the possibilities of elongation of the membrane; the external connection, between the external electric connection means and the external end of the radial excrescence, is made delicate by the fact that the radial excrescence is a fragile piece, having the same very small thickness as the rest of the membrane, so that it is not possible to provide an external electric connection using standard tags.